Variable capacity circuit for dielectric heating apparatus



L. c. AICHER ETAL Filed Aug. 1, 1946 May 6, 1952 VARIABLE CAPACI Y CIRCUIT FOR DIELECTRIC HEATING APPARATUS A ORNEY Patented May 6, 1952 VARIABLE CAPACITY CIRCUIT FOR DIELEC- TRIC HEATING APPARATUS Louis C. Aicher, Wauwatosa, Wis., and Fred J.

Vogel, Oak Park, Ill., assignors to Allis-Chalmers Manufacturing Company, Milwaukee, Wis., a corporation of Delaware Application August 1, 1946, Serial No. 687,738

8 Claims. 1

This invention relates in general to improvements in methods of molding insulating material and more particularly to methods and apparatus relating to forming articles of cellulosic material having a substantially uniform wall thickness.

The manufacture of electrical apparatus such as transformers and dynamoelectric machines requires the use of elements of insulating material which have a substantially uniform wall thickness and are adapted to be made by deforming raw material in sheet form. Where insulation is protected against moisture the raw material may be a laminated cellulosic material such as fullerboard. To make insulating elements in which the raw material is to be considerably deformed from the flat sheet form the fullerboard, or other raw material, may first be soaked in a suitable vaporizable softening liquid such as water or alcohol to render it more pliable. The wet material is then pressed between heated dies to cause it to assume the desired shape and to expel the water or other tensity and frequency is established betweenthe dies, the wet fullerboard may become heated throughout to a relatively uniform temperature by the production of dielectric losses therein, so that the heating rate may be accelerated to a considerable extent without locally overheating the material. The dies nevertheless should be brought to substantially the temperature of ebullition of the softening liquid, which will be assumed hereinafter to be water, so as not to chill the superficial portions of the material. To eliminate the necessity of separate preheating means for the dies it is advantageous to preheat the dies by applying an alternating electric field to a previously molded article or to a body of other dry insulating material inserted; between the dies.

As the dies and the material contained therebetween constitute a variable capacitive load for the oscillator, it is advantageous to connect an adjustable capacitor in series therewith to permit tuning the circuit of the dies to resonance for the frequency of the voltage of the oscillator. If necessary, an inductive conductor may be connected across the dies and the capacitor to assist the capacitor in bringing the circuit into resonance. After the article being made reaches the temperature of ebullition of water the voltage impressed on the dies may be reduced to prevent production of steam under excessive pressure within the article. It is possible to mold several articles simultaneously in dies connected in parallel in the circuit because the production of heat within each article slows up in the course of its drying, so that the moisture content tends to reach the desired value simultaneously in the different articles.

It is therefore an object of the present invention to provide an improved method of molding articles of insulating material by dielectric heating to obtain the supply of a maximum current from a given source through the circuit of the molding dies.

Another object of the present invention is to provide an improved system for molding insulating material by dielectric heating in which the circuit of the molding dies may be tuned to resonance with a minimum radiation of electromagnetic waves.

Objects and advantages other than those above set forth will be apparent from the following description when read in connection with the accompanying drawing in which:

Fig. 1 diagrammatically illustrates a system comprising two hydraulic presses and a source of alternating current for simultaneously molding two articles of insulating material from wet sheet stock by dielectric heating;

Fig. 2 is an enlarged view in cross section through a. pair of dies shown installed in a press in Fig. 1; and

Fig. 3 is a view in cross section through the dies taken along line III-III in Fig. 2.

Referring more particularly to the drawing by characters of reference, numerals 6 and I designate a pair of dies for molding articles from laminated cellulbsic insulating material such as fullerboard. Dies 6, I may have any suitable size and shape depending on the size and shape of the articles to be made, and it will be assumed that the dies are adapted to mold a piece of curved insulating channel 8. The dies are made at least partly of a suitable conductive material, and the conductive portions thereof may either be in contact with the material to be molded or be embedded in the insulating material. Satisfactory results have been consistently obtained with dies made of steel.

To reduce the cost of the dies required for molding sets of channels having the same curvature but different widths or thicknesses, die 6 may be built up of end plates 9 machined to provide the desired radius at the juncture of the web with the flanges of the channel, and of spacer plates 13 of different thicknesses of which a suitable number may be stacked between the end plates to match the desired outer profile of the channel. Die 1 may likewise be built up of end plates I2 and spacer plates l3 to match the desired inner profile of the channel. The portions of the dies which are not in contact with channel 8 are so shaped as to permit impression of a substantial voltage therebetween without flash-over and as to provide a path for the escape of steam from the channel. Ports l4 may also be provided in the dies to relieve excessive steam pressures which may build up within the channels.

Dies 6, 1 may be supported and relatively displaced by any suitable means such as a hydraulic press [5. One of the dies, die 6 for example, is mounted on press l5 through a block of suitable insulating material I6 such as Bakelite. The different component parts of die 1 may be mounted on a stand l1 resting in a tray 18 for collecting water expelled from channel 8 and discharging the collected water into a drain [9. The ram of press I5 may be actuated by means of suitable liquid supplied thereto from a reservoir 21 through a pump 22 and a four-way valve 23. Pump 22 may be driven by any suitable means such as an electric motor 24. The pressure exerted on the ram on at least the downstroke may preferably be ascertained by means of a pressure gauge 25. A gauge block 26 may be provided for ascertaining the spacing between the dies and, if desired, for serving as a stop to limit the travel of die 6.

A voltage of suitable magnitude and frequency may be impressed between dies 6, 1 from any suitable known oscillator, generally designated 21. The major portion of the connections between the dies and the oscillator preferably consists of coaxial conductors 28, 29 of relatively low impedance. The portion of the load circuit of oscillator 21, consisting of dies 6, 1 and the connections between the dies and the coaxial conductors, is preferably arranged to be resonant at the frequency impressed thereon and to radiate electromagnetic waves to the lowest possible extent. The resonant condition of the circuit may be established by means of a variable capacitor 3| connected between conductor 28 and die 6 or otherwise inserted in series in the circuit through the conductive shaft 30 of the movable capacitor plate. The shaft may be rotated through an insulated extension 40 thereof. A dial 32 associated with a pointer mounted on the shaft extension permits setting the plate in any preselected position. If necessary, an inductive conductor 33 forming a loop, may be connected between conductors 28, 29 to produce a coarse tuning of the circuit, fine tuning then being effected by adjustment of capacitor 3|. The tuned condition of the circuit may be ascertained by means of an ammeter in the plate circuit of the oscillator or by means of an ammeter 34 connected in series with the dies.

Oscillator 21 may be supplied from any suitable current supply such as a circuit 35 energized from a suitable alternating current generator (not shown). The oscillator proper or the connections of circuit 35 therewith may include any suitable known voltage regulating means such as a transformer 36 provided with a tap changer 31. The connections between transformer 36 and circuit 35 may be interrupted by means of a switch 38. Operation of tap changer 31 and of switch 38 may be controlled manually or by means of a timer, generally designated 39. If the output of oscillator 21 is sufficiently large, dies 6, 1 may be connected in parallel with one or more additional pairs of dies such as dies Al, 22, to permit simultaneous molding of a plurality of articles.

In operation, assuming that an article is to be molded in dies 6, 1, the dies are preferably first heated to substantially the boiling point of water. Such heating is conveniently effected by inserting between the dies a previously molded dry channel or dry strips of fullerboard, closing the dies, and closing switch 38 to cause an oscillator to impress ah alternating voltage on the dies. If the output voltage of oscillator 21 has a value of the order of several thousand volts and a frequency of the order of two to thirty megacycles per second, the insulating material in serted between dies 6, 1 gradually heats up by production therein of dielectric losses. The heat generated in the material is transmitted to dies 6, 1 by conduction, and the operation is continued until the dies have reached the desired temperature.

While the dies are thus being heated or while a channel is already in process of being molded in the dies, a sheet of fullerboard of suitable size and thickness is placed to soak in a tray of water. The sheet absorbs a considerable amount of water and increases in volume while becoming more pliable. The sheet may further be kneaded by hand to loosen the bond between the plies thereof and thereby facilitate subsequent deformation of the sheet within the dies.

Switch 38 is opened and the heated dies are separated, and the material previously confined therebetween is removed. The dies are then preferably coated with a film of liquid lubricating material insoluble in water to prevent sticking of the fullerboard therein and to reduce corrosion of the dies. Such liquid film may be obtained by spraying the dies with a liquid hydrocarbon or by rubbing the dies with a cake of paraffin, which melts at a temperature well below the boiling point of water.

The softened sheet of fullerboard is then laid on die 1, preferably with the fibers of the fullerboard directed across the die to reduce the danger of tearing the sheet when the dies are closed. The sheet is shaped by hand over die 1, the loosened structure of the sheet enabling the plies thereof to yield and to slide relatively to each other so as to permit the sheet to be applied against die 1 substantially without wrinkles.

Valve 23 is then actuated to cause the ram of press It to force die 6 on die 1. The sheet of fullerboard is thus compressed between the dies and a large portion of the water absorbed by the sheet is thus expelled, collected in tray l8 and discharged to drain I 9. The sheet, being confined between the dies, is thereby molded to the desired shape. Valve 23 may be so actuated as to admit fluid from pump 22 to the upper cylinder of press I 5 until gauge 25 registers a predetermined pressure which has previously been found adequate for compressing the web of channel 3 to the desired thickness.

The ram may also be caused to drop until die 6 abuts a gauge block or stop 26 resting on base I1. If block 26 is of insulating material the block may be left in place during the dielectric heating operation. If block 26 is made of metal it must be removed before application of voltage to dies 6, I. For the latter purpose it may be convenient to observe the pressure indicated by gauge 25 at the moment of engage ment of die 6 with block 26, release the pressure slightly by movement of valve 23 to permit die 6 to rise, remove block 26 and bring the pressure to the previously observed value to return die 6 to the position at which it engaged block 26.

Switch 38 may then be reclosed to cause oscillator 21 to impress an alternating voltage on the dies and thereby establish an alternating electric field in channel 8. Capacitor 3| is adjusted to the value rendering the die circuit resonant, and tap changer 31 or other voltage regulating device is adjusted manually or by means of timer 39 to cause impression on the die of a relatively high voltage to raise the temperature of channel 8 to the boiling point of water at the highest possible rate. When channel 8 reaches the boiling point of water, the remainder of the water absorbed thereby during the soaking operation gradually escapes between the dies in the form of steam.

The water content of channel 8 thus gradually decreases, whereby the capacity introduced by channel 8 in the output circuit of oscillator 21 gradually decreases. Capacitor 3| therefore should be readjusted at frequent intervals during the heating process to maintain the balance between the inductance and capacitance of the die circuit. By restoring the resonant condition of the die circuit, the operation of oscillator 2'! 4 is maintained at its highest efficiency and the flow of current through the dies is increased to the maximum value consistent with the adjustment of tap changer 31.

As channel 8 is confined between dies 6, I over the greatest portion of its surface, the escape of steam therefrom can take place only over a relatively small area. The rate of vaporization of the water contained in channel 8 should therefore be regulated to prevent building up of excessive pressures within the channel. Such regulation may be effected by actuation of tap changer 31 either manually or by timer 39 in accordance with a previously determined schedule.

The heating operation may be terminated when the water absorbed by the fullerboard during the soaking operation has been substantially entirely expelled and the moisture content of channel 8 has returned to substantially the normal value for dry fullerboard, which is in the neighborhood of six per cent at room temperature. Channels of a uniform size require a substantially uniform heating time, and opening of switch 38 to terminate the heating operation may therefore be effected by timer 39. Switch 38 may also be opened when return of channel 8 to the dry state is evidenced by the particular adjustment of capacitor 3| required for bringing the circuit of the dies back to the tuned condition.

Valve 23 may then be actuated to cause the ram of press I5 to separate dies 6, l and channel 8 may be removed from the dies. Another sheet of wet fullerboard may then be inserted in the dies for forming another channel in the manner above set forth. It will be understood however that the different steps of the process may be performed either in the exact sequence indicated 6 above or in any other order that may be found convenient or advantageous.

When two or more pairs of dies such as dies 6, I and M, 42 are connected in parallel, the dies are preferably heated simultaneously in the manner above set forth. The heated dies are then charged with wet sheets of fullerboard and reclosed. The different dies have the electrical properties of a plurality of parallelcapacitors and the die circuit may again be tuned by adjustment of the variable capacitor common thereto. During the drying operation, the rate of heat production in each channel gradually decreases to an extent determined by the decrease in the water content of the channel. The channel which has the higher water content therefore heats faster than the channel having the lower water content. The heating rate of the different channels thus adjusts itself automatically in such manner that the different channels heat at substantially the same rate and that the moisture content of the channels reaches the desired value substantially simultaneously in different channels.

Although but one embodiment of the present invention has been illustrated and described, it is apparent to one skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention orfrom the scope of the appended claims. Features disclosed but not claimed herein are claimed in a copending application Serial No. 586,742 of Louis C. Aicher, filed July 27, 1946.

It is claimed and desired to secure by Letters Patent:

1. In apparatus for simultaneously heating a plurality of charges of insulating material by dielectric losses, the combination of a plurality of pairs of dies of electrically conductive material, independent pressure applying means for each of said pair of dies, means for insulating said pressure applying means from said dies, means for electrically connecting said pairs of dies in parallel, a common adjustable capacitor serially connected with the parallel combination of said dies, a source of alternating current connected across said capacitor and said dies to establish an alternating electric field through said charges, and means for adjusting the capacity of said capacitor.

2. In apparatus for simultaneously heating a plurality of charges of insulating material by dielectric losses, the combination of a plurality of pairs of dies of electrically conductive material, independent pressure applying means for each of said pair of dies, means for insulating said pressure applying means from said dies, means for electrically connecting said pairs of dies in parallel, a common adjustable capacitor serially connected with the parallel combination of said dies, a source of alternating current connected across said capacitor and said dies to establish an alternating electric field through said charges, an inductor connected across said capacitor and said dies, and means for adjusting the capacity of said capacitor.

3. In apparatus for heating a charge of insulating material by dielectric losses, the combination of a pair of dies of electrically conductive material, pressure applying means for said pair of dies, means for insulating said pressure applying means from said dies, said dies including escape means for the release of vapor pressure from within said dies, and means for adjusting said dies to mold various sizes of articles, an

adjustable capacitor serially connected with said dies, a source of alternating current connected across said capacitor and said dies to establish an alternating electric field through said charge, means for adjusting the capacity of said capacitor, and means for adjusting the voltage of said source.

4. In apparatus for drying a charge of moist insulating material to be heated by dielectric losses, the combination of a pair of dies of electrically conductive material, pressure applying means for said pair of dies including a stop to obtain predetermined spacing between said dies when they are closed on said material, means for insulating said pressure applying means from said dies, said dies being adjustable to vary their size and having ports for the escape of steam during the drying operation, an adjustable capacitor serially connected with said dies, means for adjusting the capacity of said capacitor, a source of alternating current connected across said capacitor and said dies to establish an alternating electric field through said charge, and means for adjusting the voltage of said source.

5. In apparatus for heating a charge of moist insulating material by dielectric losses, the combination of a pair of electrically conductive dies, pressure applying means for said pair of dies including a stop to obtain predetermined spacing between said dies when they are closed on said material, means for insulating said pressure applying means from said dies, said dies being adjustable to vary their size and having ports for the escape of steam, an adjustable capacitor serially connected with said dies, a source of alternating current connected across said capacitor and said dies to establish an alternating electric field through said charge, a switch for disconnecting said source from said capacitor and said dies, and timing means for varying the voltage of said source during heating of said charge and for causing opening of said switch after a predetermined heating time.

6. In apparatus for heating a charge of moist insulating material by dielectric losses, the combination of a pair of electrically conductive dies, pressure applying means for said pair of dies including a stop to obtain predetermined spacing between said dies when they are closed on said material, means for insulating said pressure applying means from said dies, an adjustable capacitor serially connected with said dies, a source of alternating current connected across said capacitor and said dies to establish an alternating electric field through said charge, and means for adjusting the capacity of said capacitor.

7. In apparatus for heating a charge of moist insulating material by dielectric losses, the combination of a pair of dies of electrically. conductive material, pressure applying means for said dies, means for insulating said pressure applying means from said dies, said dies including escape means for the release of vapor pressure from within said-dies, an-adjustable capacitor serially connected with said dies, a source of alternating current connected across said capacitor and said dies to establish alternating'electric field through said charge, and means for adjusting the capacity of said capacitor.

8. In apparatus for simultaneously heating a plurality of charges of moist insulating material by dielectric losses, the combination of a plurality of pairs of dies'of electrically conductive material, independent pressure applying means for each of said pairs of dies, means for insulating said pressure applying means from said dies, said dies having escape means for the release of vapor pressure from within said dies, means for electrically connecting said pairs of dies in parallel, a common adjustable capacitor serially connected with the parallel connection of said dies, a source of alternating current connected across said capacitor and said dies to establish an alternating electric field through said charges, means for adjusting the capacity of said capacitor, a switch for disconnecting said source from said capacitor and said dies, and timing means for varying the voltage of said source during heating of said charges and for causing opening of said switch after a predetermined heating time.

LOUIS C. AICHER.

FRED J. VOGEL.

REFERENCES CITED The following references are of record in the file of this patent:

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